How people make perceived message effectiveness (PME) judgments remains mostly unexplored. This study assessed whether people need to spontaneously think about message effectiveness to report the message as effective on rating scales and investigated emotions as precursors to PME. After viewing one of four e-cigarette prevention messages, 1,968 adult current and former smokers and nonsmokers freely expressed thoughts about the messages in an open-ended question and answered close-ended PME items. Four expressed PME variables (positive message perceptions, negative message perceptions, positive effect perceptions, and negative effect perceptions) were coded (1 = present, 0 = absent) in the open-ended responses, and all were significantly associated with measured PME. Positive and negative emotions predicted both expressed and measured PME. Negative message perceptions was the only expressed PME construct that mediated the relationships between emotions and measured PME and outcomes (perceived risk and behavioral intentions). This suggests that messages may not need to induce effectiveness thoughts to be reported as effective, but thoughts of message ineffectiveness are a useful indicator deserving further research.